Hereinafter, a Power Saving Class (PSC) mode of a mobile station will be briefly described.
In a broadband wireless access system based on the IEEE 802.16e system, a PSC mode (for example, a sleep mode or an idle mode) for minimizing power consumption of a mobile station is supported. In the PSC mode, the operation of the mobile station is performed by the repetition of a sleep interval and a listening interval. The length of the sleep interval determined by a sleep window value and the length of the listening interval determined by a listening window value have different values according to traffic characteristics prescribed in a corresponding mobile station. Therefore, the mobile station may have the following three PSC modes according to the traffic characteristics which are currently prescribed.
That is, the mobile station is able to support Power Saving Mode Class of type 1, Power Saving Mode Class of type 2 and Power Saving Class of type 3.
The Power Saving Mode Class of type 1 is a class for a non-real-time variable rate (nrt-VR) service, transfer rate of which is variable, or a Best Effect (BE) having conventional Internet traffic characteristics. The Power Saving Mode Class of type 1 is operated by defining an initial sleep window, a final window base, a final window exponent, a listening window, and a start frame number for a sleep window.
The Power Saving Mode Class of type 2 is a class for a VoIP service or a real-time variable rate (rt-VR) service, transfer rate of which is variable. The Power Saving Mode Class of type 2 is operated by defining an initial sleep window, a listening window, and a start frame number for a sleep window.
The Power Saving Mode Class of type 3 is a class for a management message (e.g., DCD/UCD and MOB_NBR-ADV) which must be periodically transmitted to a mobile station in a power saving mode or multicast-transmission data. The Power Saving Mode Class of type 3 is operated by defining a final window base, a final window exponent, and a start frame number for a sleep window.
Hereinafter, a data retransmission method which is generally used at a transmission end and a reception end will be briefly described.
In the wireless access system, a high-speed data service is provided using restricted resources. An automatic retransmission request method for efficiently using resources is used. That is, if transmission failure occurs during data transmission, a reception end requests data retransmission. At this time, an Automatic Repeat reQuest (ARQ) scheme is generally as the automatic retransmission request scheme.
In the ARQ scheme, after a reception end receives data, the reception end notifies a transmission end of whether or not the reception end has successfully received data through an Acknowledgement/Non-Acknowledgement (ACK/NACK) signal, and the transmission end retransmits data related to the signal when a NACK signal is received. The ARQ scheme includes three schemes: a Stop-And-Wait (SAW) ARQ scheme, a Go-Back-N (GBN) ARQ scheme and a Selective-Repeat (SR) ARQ scheme.
If data is transmitted in a packet format, a high data rate is required. Accordingly, a coding rate or a modulation method of a level corresponding to the high data rate has also been applied to the communication system in order to prevent errors occurring in high-speed transmission environments. In addition, there has been a need to provide an ARQ scheme suitable for high-speed transmission environments. Thus, a Hybrid ARQ (HARQ) scheme has been suggested.
In the ARQ scheme, information is discarded when an error occurs in the information. However, in the HARQ scheme, the receiving end stores information in which an error has occurred in a buffer and then combines the stored information with information for retransmission to apply Forward Error Correction (FEC). That is, the HARQ scheme can be considered as a combination of the ARQ scheme with FEC. The HARQ scheme may be roughly divided into the following four types.
In the first type of HARQ scheme, the receiving end always checks an error detection code included in data to preferentially apply the FEC scheme. When a received packet contains an error, the receiving end requests that the transmitting end retransmit the received packet. The receiving end discards the erroneous packet and the transmitting end uses the same FEC code as that of the discarded packet to retransmit the packet.
The second type of HARQ scheme is referred to as an “Incremental Redundancy (IR) ARQ scheme”. In the IR ARQ scheme, the receiving end stores an initially transmitted packet in a buffer without discarding the packet and then combines the stored packet with redundancy bits included in the retransmitted packet. During retransmission, the transmitting end retransmits only parity bits excluding data bits. The transmitting end uses different parity bits every retransmission.
The third type of HARQ scheme is a special case of the second type. Each packet is self-decodable. When the transmitting end performs retransmission, the transmitting end constructs and retransmits a packet including both data and an erroneous portion. Although this scheme enables more correct decoding than the second type, the efficiency of coding gain is low.
The fourth type of HARQ scheme provides a function to store data initially received by the receiving end and combine the stored data with retransmitted data in addition to the functions of the first type. The fourth type of HARQ scheme is also referred to as “metric combining” or “chase combining”. The fourth type of HARQ scheme has an excellent Signal to Interference Noise Ratio (SINR) and always uses the same parity bits of data to be retransmitted.
In the wireless access system, ARQ and HARQ may be simultaneously used. Data to be transmitted by the transmitting end with the ARQ is transferred to the receiving end with the HARQ through the HARQ of the transmitting end. Data which is successfully received by the receiving end is transferred to the receiving end with the ARQ.